Data storage systems may maintain more than one copy of data to protect against losing the data in the event of a failure of any of the data storage components. A secondary copy of data at a remote site is typically used in the event of a failure at the primary site. Secondary copies of the current data contained in the primary site are typically made as the application system is writing new data to a primary site. In some data storage systems the secondary site may contain two or more peer computers operating together as a backup appliance to store the data in one or more storage devices. Each peer computer receives inbound data from the primary site and transfers the data to a storage controller, storage device(s), or other computers for backup storage of the data. This type of system could be used for a disaster recovery solution where a primary storage controller sends data to a backup appliance that, in turn, offloads the transfers to a secondary storage controller at a remote site. In such backup systems, data is typically maintained in volume pairs. A volume pair is comprised of a volume in a primary storage device and a corresponding volume in a secondary storage device that includes an identical copy of the data maintained in the primary volume. Typically, the primary volume of the pair will be maintained in a primary direct access storage device (DASD) and the secondary volume of the pair is maintained in a secondary DASD shadowing the data on the primary DASD. A primary storage controller may be provided to control access to the primary storage and a secondary storage controller may be provided to control access to the secondary storage.
The backup appliance typically receives data transfers for specific volumes from a primary storage controller. The backup appliance maintains consistent transactions sets, wherein application of all the transactions to the secondary device creates a point-in-time consistency between the primary and secondary devices. For each consistent transactions set, there will be one data structure created that will contain information on all outbound transfers in the set. This structure will be maintained on both of the peer nodes of the backup appliance. The backup appliance will maintain consistent transactions sets while offloading the transactions sets to the secondary device asynchronously. Both peer nodes may transfer the data to any of the storage devices. To obtain the shortest transfer time it is necessary to divide the data transfers between the peers. An equal division of the data transfers between the two peers may not be optimal because the latency time to transfer data to a particular storage device may be different for each peer. This may result in one peer finishing before the other, and idle time for the other peer. A division of the data transfers between the two peers that results in both peers finishing simultaneously would reduce the total throughput latency for the system because both peers have to finish transferring the current consistent transactions set before beginning to transfer the subsequent one. There is a need to divide the data transfers between the peers to achieve an optimal minimum transfer time to transfer all of the data.